1. Field of the Invention
This invention relates generally to ADSL line driver circuits, and more particularly to an actively biased class AB amplifier output stage to achieve very low quiescent current for an ADSL line driver.
2. Description of the Prior Art
In an application such as an ADSL central office, where the communication signal has a large crest factor, line drivers are used to provide the requisite large line currents generally associated with low distortion output stages having large output voltage swings. A conventional low distortion power amplifier class xe2x80x98ABxe2x80x99 output stage 100 such as illustrated in FIG. 1 provides a good output swing and has an output current that is limited to the bias current xe2x80x98Ixe2x80x99 multiplied by the Hfe of, for example, an NPN source transistor 102 or a PNP sink transistor 104. Such amplifiers are common in ADSL communications since they represent a good balance between distortion and quiescent power (power dissipated during idle periods). To accommodate the large crest factors associated with ADSL communications, the class xe2x80x98ABxe2x80x99 output stage depicted in FIG. 1 is generally modified as illustrated in FIG. 2 to achieve a current gain that is the product of two transistor gains (Hfe1*Hfe2) as contrasted with the single transistor Hfe current gain associated with the amplifier of FIG. 1. In this way, the ratio of signal current to quiescent current is significantly increased, thereby significantly increasing amplifier efficiency. The amplifier 200 shown in FIG. 2, however, is problematic in that it significantly increases the amount of supply voltage headroom required to operate the amplifier for a given output voltage. Since the amplifier 200 is required to be differential to support ADSL, the headroom necessary to operate the amplifier 200 will be approximately two times the voltage drop across the current source 202 (typically 2*0.2V=0.4V) plus four transistor Vbe voltage drops (typically 4*0.9V=3.6V) required by the two amplifier output transistors 204, 206, as well as two additional amplifier output transistors (not shown). The available supply voltage typically then will be reduced by 4.0 volts which is unacceptable for a system having only a 15 volt supply, for example, since only 11 volts would be available to support the large voltage swings necessary to support ADSL communications.
In view of the foregoing, a need exists for an ADSL line driver amplifier that has high drive capability in combination with lower quiescent current drain and more efficient use of available supply voltage than currently available using conventional class xe2x80x98ABxe2x80x99 line driver output stages.
To meet the above and other objectives, the present invention provides an actively biased class xe2x80x98ABxe2x80x99 amplifier output stage suitable for supporting ADSL applications. The actively biased class xe2x80x98ABxe2x80x99 amplifier output stage reduces the quiescent current required in low distortion amplifiers. The actively biased class xe2x80x98ABxe2x80x99 amplifier output stage further provides high drive capability from low quiescent current while simultaneously achieving efficient utilization of available supply voltage.
According to one embodiment, an input signal is applied to a bias circuit having a load resistor RB as well as the input of the amplifier output stage connected in a unity gain configuration. The load resistor RB is connected at the output of the bias circuit to set the boost current IBOOST. The current available to drive the amplifier output stage transistors is now modulated in such a way that it is proportional to the collector current demanded by the load (RLOAD). The load resistor RB can be a resistor or a combination of resistor and/or capacitor and/or inductor in series or parallel to boost the bias current in such a way as to satisfy the current requirements of the load (RLOAD). A capacitor in parallel to RB also provides bias to drive parasitic capacitance of the output transistors in a high slew condition.
In one aspect of the invention, a class xe2x80x98ABxe2x80x99 amplifier output stage is biased that exhibits very low distortion.
In another aspect of the invention, a class xe2x80x98ABxe2x80x99 amplifier output stage is implemented such that the current available to drive the output transistors is modulated in such a way that it is proportional to the collector current demanded by the load.
In yet another aspect of the invention, a class xe2x80x98ABxe2x80x99 amplifier output stage is implemented that has high drive capability in combination with lower quiescent current drain and more efficient use of available supply voltage than currently available using conventional class xe2x80x98ABxe2x80x99 line driver output stages.
In still another aspect of the invention, a class xe2x80x98ABxe2x80x99 amplifier output stage is implemented that provides enhanced power efficiency for an ADSL central office line driver where the communication signal has a large crest factor.